View source for Angular distance

The '''angular distance''' (or '''angular separation''') between two [[Object|objects]] tells how much two objects are moving in the same direction. It is usually denoted '''ΔR''' ('''delta R''').

== At hadron colliders ==
At [[Hadron collider|hadron colliders]], the angular distance is usually defined in a way that is [[Longitudinal boost invariance|invariant under longitudinal boosts]]. This is achieved by using the [[pseudorapidity]] difference <math>\Delta\eta</math> of the objects rather than an actual angle. In the [[transverse plane]], the momentum of the [[initial state]] is ([[Primordial kt|essentially]]) zero, so the [[azimuthal angle]] difference <math>\Delta\phi</math> can be used directly. This gives the definition:

<math>\Delta R = \sqrt{(\Delta\eta)^2 + (\Delta\phi)^2}.</math>


== At lepton colliders ==
At [[Lepton collider|lepton colliders]], the vector angle between the object momenta may be used directly.